Will try to be on time (5)
I believe the answer is:
heart
'will' is the definition.
(thesaurus)
'try to be on time' is the wordplay.
'try' becomes 'hear' (as in to try someone in court or hear their case).
'to be on' is a charade indicator (letters next to each other).
'time' becomes 't' (abbreviation).
'hear'+'t'='HEART'
(Other definitions for heart that I've seen before include "Playing card - pity" , "See 22" , "Earth (anag) -- compassion" , "Pumping organ" , "the name of a prison in Midlothian!" .)